This proposal is one of three projects forming an Interactive ROI application entitled "Anti-cancer mechanisms of the soy isoflavone genistein", the principle objective of which is to determine the mechanism(s) by which the trihydroxyisoflavone genistein exerts its chemopreventive effect against several types of cancer. Genistein and the dihydroxyisoflavone daidzein are present in large quantities (2-3 mg/g) in soy, but are not found in other foods in the American diet. Women who eat a diet rich in soy foods have a much lower incidence of breast cancer than those who eat a typical American diet. Although genistein has long been known for its weak estrogenic properties, it has also been shown in vitro to be an inhibitor of many protein tyrosine kinases and to inhibit the growth of many cancer cell lines, suggesting that it may have an important role in the prevention of cancer. However, the crucial cellular targets of genistein in its role as an anti-proliferative agent have not yet been identified. The specific aims of this proposal are to determine: (1) the inhibitory effects of genistein on the serum-,estradiol- and EGF- stimulated growth of a human breast cancer cell line with and without expressed estrogen receptor; (2) whether genistein in human breast cancer cell lines stimulated by growth factors inhibits the phosphorylation of (a) the estrogen receptor, (b) the EGF receptor; (c) key kinases in the signal transduction sequence activated by EGF; (3) whether genistein suppresses the mitogenic induction of expression of the immediate early genes c-fos, c-jun and c-myc; (4) in a rat model of breast cancer, whether dietary genistein has a larger effect when limited to the period from 25 days of age up to and during the administration of the chemical carcinogen at 50 days of age (anti-initiation effect), or after the first mammary tumors have begun to appear (progression-promotion effect); (5) whether genistein administered during puberty in female rats alters cell differentiation and proliferation in the mammary; and (6) whether inclusion of genistein in the diet from puberty to time of administration of DMBA decreases the rate of formation and amount DMBA-DNA adducts in the mammary. It is intended that this project should establish a rationale basis for the use of soy as a recommended foodstuff to lower the risk of cancer in Americans.